Systems have been proposed for providing variation of camshaft condition, particularly for vehicles, so as to enhance the operating characteristics of the engine. In one example, a camshaft has dual profiles giving respective low and high inlet valve lift, and thus allowing a range of valve timing to enhance engine economy and performance. The relevant profile is selected, typically by a microprocessor based engine control unit (ECU), according to driver demand and other relevant parameters.
Many arrangements for switching cam profile have been proposed. One solution provides a dual tappet for each inlet valve. Each tappet operates on a low lift cam profile when engine is operated at low loads, but when engine is operating at high loads a high lift mode can be activated by a high pressure oil feed to engage a high lift cam profile, on demand.
The ECU controls other vehicle systems, particular systems for minimizing exhaust emissions, and for that purpose a diagnostic is required to confirm that the desired camshaft condition has been effected. In the example given above, it is not sufficient merely to monitor the presence or absence of a high pressure oil supply to a tappet, since the tappets themselves may malfunction.
It has accordingly been proposed to monitor the constituents of the engine exhaust, in particular the oxygen content, to confirm that the measured value lies within a range associated with the demanded camshaft condition. The ECU may for example include a look-up table giving the corresponding values for each camshaft condition by reference to fuelling and/or engine rpm.
U.S. Pat. No. 7,621,175 discloses a method of detecting degradation of a cam profile switching system in a Vee engine, which method compares the exhaust constituents of the two cylinder banks during a forced change of cam condition for one bank of the engine. If, for example, the oxygen concentration in the two exhaust streams is unchanged, it may be concluded that the cam profile switching system has degraded in the bank subjected to the forced change. Typically the alternative cam condition has not been adopted due to a malfunction in an electrical or hydraulic circuit.
In this arrangement, the forced change can be applied periodically to each bank, and for a short time during a suitable engine operating condition.
It would be advantageous to have an alternative diagnostic able to detect correct functioning of a varied camshaft condition.